It’s the last thing public health officials want to see in the midst of a pandemic: more than two months after pharmaceutical giant AstraZeneca and Oxford University scientists released their COVID-19 vaccine, countries in Europe and elsewhere are pausing its use amid disconcerting reports that a small number of recipients have experienced blood clots, some of them fatal. If that weren’t enough, a new study also showed the vaccine was not effective in protecting people from a variant of the COVID-19 virus that originated in South Africa and is slowly gaining ground in other parts of the world.

In the report published in the New England Journal of Medicine, scientists in South Africa found that two doses of the AstraZeneca vaccine were about 22% efficacious in protecting people from getting sick sick with COVID-19 from the new variant of the virus, known as B.1.351. While there were no severe cases of COVID-19 report among the more than 2000 people in the study, that efficacy is not sufficient enough protection since similar percentages of people who got the vaccine as who received the placebo became sick. That means that the utility of the AstraZeneca vaccine, including against B.1.351, needs to be carefully weighed against the burden of the pandemic to determine if the vaccine is worth using in areas where the variant is spreading quickly.

More immediately, the company is also contending with continued fallout from reports of blood clots among people who have been vaccinated with its shot. The European Medicines Agency, which oversees drugs and vaccines in Europe, is expected to issue a guidance on March 18 about whether the side effects were related to the vaccine after reviewing these reports; in the meantime, it says the benefits of the shot outweigh any potential risks. Despite that reassurance, European countries including France, Germany, Italy and more have suspended vaccinations with the AstraZeneca shot. Venezuela, which has not yet authorized the AstraZeneca vaccine, now says it will not do so. “What we’re seeing is a domino effect,” says Dr. Anna Durbin, professor of international health at Johns Hopkins Bloomberg School of Public Health. “Nobody wants to be the country that didn’t stop [vaccinating] and then find out that the side effects are really related [to the vaccine].”